1. Field of the Invention
The present invention relates to a resistor element whose electrical resistance depends on temperatures. The resistor element is suitably used in a thermal flowmeter for measuring a flow rate of a fluid in a passage. The thermal flowmeter may be used in intake air introduced into an internal combustion engine. In these applications, the resistor element needs to have quick response and durability at high temperatures.
2. Description of Related Art
A resistor element used in a thermal flowmeter has a metallic resistor having an electrical resistance varying with temperature. The resistor element has a substrate, a metallic resistor supported by the substrate, a pair of leads for electrically connecting the metallic resistor to a circuit, and a glass layer coated onto the metallic resistor so as to protect the metallic resistor. The metallic resistor may be a film coated onto the substrate. Alternatively, the metallic resistor may be a wire that is wound around the substrate. The metallic resistor may be composed of platinum or an alloy including platinum.
The glass layer is made of a glass having a high thermal conductivity so as to ensure quick response of the metallic resistor to temperature changes. The glass layer protects the metallic resistor so that the metallic resistor does not corrode, abrade or change its electrical resistance even at high temperatures.
FIG. 3 shows a glass layer of a conventional resistor element. The resistor element has a ceramic substrate 22, a metallic film 24 coated onto a surface of the ceramic substrate 22, and a glass layer 25 coated onto the metallic film 24.
The glass layer 25 may have a bubble 28 therein due to the manufacturing process. The bubble in the glass layer does not conduct much heat so that the response of the metallic resistor to temperature changes is delayed. In the process of making the resistor element, a slurry including a glass and a binder is coated onto the metallic resistor, and the slurry is fired so as to form a glass layer. An organic compound may be present in the binder as an impurity. Alternatively, the organic compound may be stuck onto the metallic resistor from the beginning. During the firing step, the organic compound may become a gas, and the gas may remain trapped in the glass layer as bubbles.
In FIG. 3, the glass layer 25 has a hole 26 exposing a surface of the metallic resistor 24. The exposed surface of the metallic resistor 24 is susceptible to corrosion, abrasion and oxidation, and the metallic resistor 24 may change its electrical resistance over a long period. The glass layer 25 has a hole with a thin part 27. The part 27 may be erased by sand particles flowing with a gas, exposing the metallic resistor 24. During the step of firing the glass layer, the bubbles in the glass layer may explode, thereby forming the hole 26 and a thin part 27 in the glass layer.